Fabricating transmon qubit flip-chip structures for quantum computing devices

ABSTRACT

A quantum computing device is formed using a first chip and a second chip, the first chip having a first substrate, a first set of pads, and a set of Josephson junctions disposed on the first substrate. The second chip has a second substrate, a second set of pads disposed on the second substrate opposite the first set of pads, and a second layer formed on a subset of the second set of pads. The second layer is configured to bond the first chip and the second chip. The subset of the second set of pads corresponds to a subset of the set of Josephson junctions selected to avoid frequency collision between qubits in a set of qubits. A qubit is formed using a Josephson junction from the subset of Josephson junctions and another Josephson junction not in the subset being rendered unusable for forming qubits.

TECHNICAL FIELD

The present invention relates generally to a superconductor device, afabrication method, and fabrication system for superconducting quantumdevices. More particularly, the present invention relates to a device,method, and system for fabricating transmon qubit flip-chip structuresfor quantum computing devices.

BACKGROUND

Hereinafter, a “Q” prefix in a word or phrase is indicative of areference of that word or phrase in a quantum computing context unlessexpressly distinguished where used.

Molecules and subatomic particles follow the laws of quantum mechanics,a branch of physics that explores how the physical world works at afundamental level. At this level, particles behave in strange ways,taking on more than one state at the same time, and interacting withother particles that are very far away. Quantum computing harnessesthese quantum phenomena to process information.

The computers we use today are known as classical computers (alsoreferred to herein as “conventional” computers or conventional nodes, or“CN”). A conventional computer uses a conventional processor fabricatedusing semiconductor materials and technology, a semiconductor memory,and a magnetic or solid-state storage device, in what is known as a VonNeumann architecture. Particularly, the processors in conventionalcomputers are binary processors, i.e., operating on binary datarepresented in 1 and 0.

A quantum processor (q-processor) uses the odd nature of entangled qubitdevices (compactly referred to herein as “qubit,” plural “qubits”) toperform computational tasks. In the particular realms where quantummechanics operates, particles of matter can exist in multiplestates—such as an “on” state, an “off” state, and both “on” and “off”states simultaneously. Where binary computing using semiconductorprocessors is limited to using just the on and off states (equivalent to1 and 0 in binary code), a quantum processor harnesses these quantumstates of matter to output signals that are usable in data computing.

Conventional computers encode information in bits. Each bit can take thevalue of 1 or 0. These 1s and 0s act as on/off switches that ultimatelydrive computer functions. Quantum computers, on the other hand, arebased on qubits, which operate according to two key principles ofquantum physics: superposition and entanglement. Superposition meansthat each qubit can represent both a 1 and a 0 at the same time.Entanglement means that qubits in a superposition can be correlated witheach other in a non-classical way; that is, the state of one (whether itis a 1 or a 0 or both) can depend on the state of another, and thatthere is more information that can be ascertained about the two qubitswhen they are entangled than when they are treated individually.

Using these two principles, qubits operate as more sophisticatedprocessors of information, enabling quantum computers to function inways that allow them to solve difficult problems that are intractableusing conventional computers. IBM has successfully constructed anddemonstrated the operability of a quantum processor usingsuperconducting qubits (IBM is a registered trademark of InternationalBusiness Machines corporation in the United States and in othercountries.)

A superconducting qubit includes a Josephson junction (Josephsonjunction). A Josephson junction is a superconducting tunnel junction,which is formed by separating two thin-film superconducting metal layersby a non-superconducting material. When the metal in the superconductinglayers is caused to become superconducting—e.g. by reducing thetemperature of the metal to a specified cryogenic temperature—pairs ofelectrons can tunnel from one superconducting layer through thenon-superconducting layer to the other superconducting layer. Othermethods of forming Josephson junctions exist and this description is notmeant to be limiting. In a qubit, the Josephson junction—which functionsas a dispersive nonlinear inductor—is electrically coupled in parallelwith one or more capacitive devices forming a nonlinear microwaveoscillator. The oscillator has a resonance/transition frequencydetermined by the value of the inductance and the capacitance in thequbit circuit. Any reference to the term “qubit” is a reference to asuperconducting qubit circuitry that employs a Josephson junction,unless expressly distinguished where used.

In a superconducting state, the material firstly offers no resistance tothe passage of electrical current. When resistance falls to zero, acurrent can circulate inside the material without any dissipation ofenergy. Secondly, the material exhibits the Meissner effect, i.e.,provided they are sufficiently weak, external magnetic fields do notpenetrate the superconductor, but remain at its surface. When one orboth of these properties are no longer exhibited by the material, thematerial is said to be in a normal state and no longer superconducting.

A critical temperature of a superconducting material is a temperature atwhich the material begins to exhibit characteristics ofsuperconductivity. Superconducting materials exhibit very low or zeroresistivity to the flow of current. A critical field is the highestmagnetic field, for a given temperature, under which a material remainssuperconducting.

Superconductors are generally classified into one of two types. Type Isuperconductors exhibit a single transition at the critical field. TypeI superconductors transition from a non-superconducting state to asuperconducting state when the critical field is reached. Type IIsuperconductors include two critical fields and two transitions. At orbelow the lower critical field, type II superconductors exhibit asuperconducting state. Above the upper critical field, type IIsuperconductors exhibit no properties of superconductivity. Between theupper critical field and the lower critical field, type IIsuperconductors exhibit a mixed state. In a mixed state, type IIsuperconductors exhibit an incomplete Meissner effect, i.e., penetrationof external magnetic fields in quantized packets at specific locationsthrough the superconductor material.

The information processed by qubits is carried or transmitted in theform of microwave signals/photons in the range of microwave frequencies.The microwave signals are captured, processed, and analyzed to decipherthe quantum information encoded therein. A readout circuit is a circuitcoupled with the qubit to capture, read, and measure the quantum stateof the qubit. An output of the readout circuit is information usable bya q-processor to perform computations.

A superconducting qubit has two quantum states—|0> and |1>. These twostates may be two energy states of atoms, for example, the ground (|g>)and first excited (|e>) state of a superconducting artificial atom(superconducting qubit). Other examples include spin-up and spin-down ofthe nuclear or electronic spins, two positions of a crystalline defect,and two states of a quantum dot. Since the system is of a quantumnature, any combination of the two states are allowed and valid.

Superconducting devices such as qubits are fabricated usingsuperconducting and semiconductor materials in known semiconductorfabrication techniques. A superconducting device generally uses one ormore layers of different materials to implement the device propertiesand function. A layer of material can be superconductive, conductive,semi-conductive, insulating, resistive, inductive, capacitive, or haveany number of other properties. Different layers of materials may haveto be formed using different methods, given the nature of the material,the shape, size or placement of the material, other materials adjacentto the material, and many other considerations.

The software tools used for designing semiconducting and superconductingdevices produce, manipulate, or otherwise work with an electrical layoutand device components on very small scales. Some of the components thatsuch a tool may manipulate may only measure few nanometers across whenformed in a suitable substrate.

A layout includes shapes whose shape and position is selected in thetool according to the device's objective. Once a design layout, alsoreferred to simply as a layout, has been finalized for a device or agroup of devices, the design is converted into a set of masks orreticles. A set of masks or reticles is one or more masks or reticles.During manufacture, a semiconductor wafer is exposed to light orradiation through a mask to form microscopic components comprising thestructures. This process is known as photolithography. A mask is usablefor manufacturing or printing the contents of the mask onto the wafer.During the photolithographic printing process, radiation is focusedthrough the mask and at certain desired intensity of the radiation. Thisintensity of the radiation combined with any materials that aredeposited using the radiation is commonly referred to as “dose”. Thefocus and the dosing of the radiation is controlled to achieve thedesired shape and electrical characteristics of structures on the wafer.

A Fabrication process for a semiconducting or superconducting deviceincludes not only dosing but other methods of depositing and/or removingmaterials having various electrical and/or mechanical characteristics.For example, a conducting material may be deposited using an beam ofions of that material; a hard insulator may be dissolved using achemical or eroded using mechanical planning. These examples ofoperations in a fabrication process are not intended to be limiting.From this disclosure, those of ordinary skill in the art will be able toconceive many other operations in a fabrication process that is usableto fabricate a device according to the illustrative embodiments, and thesame are contemplated within the scope of the illustrative embodiments.

Superconducting devices are often planar, i.e., where the superconductorstructures are fabricated on one plane. A non-planar device is athree-dimensional (3D) device where some of the structures are formedabove or below a given plane of fabrication.

Some qubits are fabricated using a flip-chip geometry. In the flip-chipgeometry, a qubit chip (also referred to as a “Qchip”) is fabricatedhaving a number of individual qubits upon a substrate, and an interposerchip having one or more connections is fabricated on a separatesubstrate. Solder bumps are deposited onto chip pads on a first surfaceof the qubit chip and/or interposer chip, and the qubit chip orinterposer chip is flipped over so that its first side faces down. Thequbit chip and interposer chip are aligned and bump-bonded, so that thesolder of the solder bumps complete the electrical connection of thequbit chip and the interposer chip.

According to the illustrative embodiments described herein, a flip-chipgeometry can combine an interposer chip with other chips containingcomponents of qubits but not the complete qubit. For example, a junctionchip (also referred to as a “J-chip”) is a chip according to theillustrative embodiments that has a plurality of individual Josephsonjunctions fabricated thereon. A Josephson junction from the plurality isusable to form a qubit when an interposer is combined with the J-chip ina flip-chip configuration in a manner described herein. According to theillustrative embodiments, an interposer chip having one or moreconnections is fabricated on a separate substrate. Bumps of a suitablematerial, such as soldering material with the desired electrical,thermal, ductility, and malleability properties in cryogenic and otheroperating conditions as described herein, are deposited onto chip padson a first surface of the J-chip and/or interposer chip. Generally, anyreference to a solder bump should be construed to include bumps made ofmaterial that satisfy these requirements. The J-chip or the interposerchip is flipped over so that a first side of the flipped chip facestowards a first face of the non-flipped chip from the two chips. TheJ-chip and interposer chip are aligned and bump-bonded, so that thematerial of a bump completes an electrical connection between the J-chipand the interposer chip.

The readout circuitry is generally coupled with a qubit byelectromagnetic resonance (usually a microwave or radio-frequencyresonance) using a resonator. A resonator in the readout circuitrycomprises inductive and capacitive elements. Some qubits arefixed-frequency qubits, i.e., their resonance frequencies are notchangeable. Other qubits are frequency-tunable qubits. A q-processor canemploy fixed-frequency qubits, frequency-tunable qubits, or acombination thereof.

The illustrative embodiments recognize that a fixed-frequency qubit isdesigned to be fixed in frequency to improve immunity to noise. Theillustrative embodiments recognize that when the resonance frequenciesof two coupled qubits on a chip are the same or within a threshold bandof frequencies, or their higher transition frequencies are on resonanceor close to resonance, then negative effects can happen such as,crosstalk, quantum decoherence, energy decay, creation of mixed states,unintended information transfer, quantum state leakage and so on. Theillustrative embodiments further recognize that such qubits can alsonegatively affect the performance or utility of certain quantum gates,such as cross-resonance gates which have stringent requirements on thespectrum of resonance frequencies of qubits upon which the gate isoperating on. The illustrative embodiments further recognize that onechallenge in quantum processors that are based on fixed-frequency qubitsis frequency crowding or frequency collision between adjacent qubits.

The illustrative embodiments recognize that another challenge in quantumprocessors that are based on fixed-frequency qubits is low On/Off ratiosbetween when microwave signals turn On an interaction (On interactionstrength) and the interactions between coupled qubits when these signalsare disabled (Off interaction strength). The illustrative embodimentsfurther recognize that yet another challenge in quantum processors thatare based on fixed-frequency qubits is enabling a gate of interestwithout producing unwanted interactions at other sites. The illustrativeembodiments further recognize that imperfections in the fabrication andthe materials used in the presently available fabrication methods forfixed-frequency qubits leads to deviations from an intended resonancefrequency.

The illustrative embodiments further recognize that in some cases, aJosephson junction of a qubit can be tuned to adjust a resonancefrequency of the qubit. However, the illustrative embodiments recognizethat once a flip-chip assembly has been formed, a Josephson junction onthe qubit chip or J-chip becomes physically inaccessible for operationssuch as laser annealing, by which the impedance of the Josephsonjunction can be modified or tuned. In such a case, the flip-chipassembly becomes fixed with a qubit configuration in which a Josephsonjunction has an undesirable impedance. Therefore, the illustrativeembodiments recognize that a flip-chip configuration is needed in whicha qubit can be formed with the desired resonance frequency even afterthe interposer has been flipped over the J-chip.

SUMMARY

The illustrative embodiments provide a method and system of fabricationfor a quantum computing device. The embodiment forms a first chip havinga first substrate, a first set of pads, and a set of Josephson junctionsdisposed on the first substrate. The embodiment further forms a secondchip having a second substrate, a second set of pads disposed on thesecond substrate opposite the first set of pads, and a second layerformed on a subset of the second set of pads, the second layerconfigured to bond the first chip and the second chip, wherein thesubset of the second set of pads corresponds to a subset of the set ofJosephson junctions selected to avoid frequency collision between qubitsin a set of qubits, a qubit in the set of qubits resulting from aJosephson junction in the subset of Josephson junctions. Thus, theembodiment provides a quantum computing device in a flip-chipconfiguration where several Josephson junctions on a J-chip are madewith manufacturing variations in properties, and only some Josephsonjunctions are selected to form qubits with desirable frequency collisionavoidance characteristics.

Another embodiment further forms an unusable Josephson junction in theset of Josephson junctions, wherein a first Josephson junction in theset of Josephson junctions is modified to become the unusable Josephsonjunction responsive to the first Josephson junction being excluded fromthe subset of the set of Josephson junctions. Thus, the embodimentprovides a quantum computing device in a flip-chip configuration whereother Josephson junctions on the J-chip are rendered unusable for use inthe quantum computing device.

Another embodiment further forms a disconnected pad in the first set ofpads, wherein the first set of pads comprises a first pad electricallycoupled to the first Josephson junction, and wherein the first pad iselectrically disconnected from the first Josephson junction to form thedisconnected pad, the disconnected pad rendering the first Josephsonjunction unusable. Thus, the embodiment provides a hardware change torender Josephson junctions on the J-chip unusable for use in the quantumcomputing device.

In another embodiment, an electrical property of the first Josephsonjunction is modified such that the first Josephson junction no longeroperates as a Josephson junction. Thus, the embodiment provides anotherhardware change to render Josephson junctions on the J-chip unusable foruse in the quantum computing device.

In another embodiment, the subset is selected based upon a measurementof a parameter associated with each of the set of Josephson junctions.Thus, the embodiment provides a property of Josephson junctions on theJ-chip based on which the junction can be selected for use in thequantum computing device.

In another embodiment, the resonance frequency associated with aparticular qubit is one member selected from a set of (i) a predictedresonance frequency calculated based upon the measured parameter, and(ii) an actual measured resonance frequency of the particular qubit.Thus, the embodiment provides a predicted property of a qubit formedusing a Josephson junctions on the J-chip, based on which the junctioncan be selected for use in the quantum computing device.

In another embodiment, the parameter includes a resistance associatedwith a Josephson junction in the set of Josephson junctions. Thus, theembodiment provides a specific property of Josephson junctions on theJ-chip based on which the junction can be selected for use in thequantum computing device.

In another embodiment, the resistance is a normal-state resistance ofthe Josephson junction. Thus, the embodiment provides a specificproperty of Josephson junctions on the J-chip based on which thejunction can be selected for use in the quantum computing device.

Another embodiment further forms a first set of protrusions formed onthe first chip. The embodiment further includes a set of bumps formed onthe first layer of the second chip, the set of bumps formed of amaterial having above a threshold ductility at a room temperature range,wherein set of bumps are configured to cold weld to the first set ofprotrusions. Thus, the embodiment provides an apparatus for detachablyconfiguring the two chips in the flip-chip configuration.

In another embodiment, the first set of protrusions is of at least onemember selected from a set comprising Gold and Platinum. Thus, theembodiment provides a material for an apparatus for detachablyconfiguring the two chips in the flip-chip configuration.

In another embodiment, the set of bumps is of at least one memberselected from a set comprising Indium, Tin, Lead, and Bismuth. Thus, theembodiment provides an apparatus for bonding configuring the two chipsin the flip-chip configuration.

Another embodiment further forms a flip-chip assembly comprising thefirst chip detachably attached to the second chip using the cold weld,wherein a parameter of a Josephson junction inside the flip-chipassembly is tunable by disassembling the flip-chip assembly at the coldweld. Thus, the embodiment provides for a detachable configuration ofthe two chips in the flip-chip configuration or adjusting a property ofa Josephson junction.

An embodiment includes a fabrication system for fabricating the quantumcomputing device.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives and advantages thereof, willbest be understood by reference to the following detailed description ofthe illustrative embodiments when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 depicts a block diagram of a network of data processing systemsin which illustrative embodiments may be implemented;

FIG. 2 depicts a qubit for use in a quantum processor;

FIG. 3 depicts an example cross-section view of a flip-chip quantumcomputing device illustrating a problem that can be solved using anillustrative embodiment;

FIG. 4 depicts a block diagram of an example J-chip configuration inaccordance with an illustrative embodiment;

FIG. 5 depicts a block diagram of an example configuration reached inthe fabrication of the flip-chip device in accordance with anillustrative embodiment;

FIG. 6 depicts a block diagram of an example configuration reached inthe fabrication of the flip-chip device in accordance with anillustrative embodiment;

FIG. 7 depicts an example graph for calculating a predicted frequency ofa qubit based upon a measured junction resistance in accordance with anillustrative embodiment;

FIG. 8 depicts a block diagram of an example configuration reached inthe fabrication of the flip-chip device in accordance with anillustrative embodiment;

FIG. 9 depicts a block diagram of an example configuration reached inthe fabrication of the flip-chip device in accordance with anillustrative embodiment;

FIG. 10 depicts a block diagram of an example flip chip configurationreached in the fabrication of the flip-chip device in accordance with anillustrative embodiment;

FIG. 11 depicts a top-down schematic view of an example J-chipconfiguration in accordance with an illustrative embodiment;

FIG. 12 depicts a block diagram of an alternative example configurationin accordance with an illustrative embodiment;

FIG. 13 depicts a block diagram of an example J-chip assembly inaccordance with an illustrative embodiment;

FIG. 14 depicts a block diagram of an example J-chip configuration inaccordance with an illustrative embodiment;

FIG. 15 depicts a block diagram of an example detachable configurationin accordance with an illustrative embodiment;

FIG. 16 depicts a block diagram of an example detachable conductivecoupling configuration in accordance with an illustrative embodiment;

FIG. 17 depicts a block diagram of another example configuration of adetachable conductive coupling in accordance with an illustrativeembodiment;

FIG. 18 depicts a flowchart of an example process for fabricating aflip-chip quantum computing device in accordance with an illustrativeembodiment;

FIG. 19 depicts a flowchart of an example process for fabricating aflip-chip quantum computing device in accordance with an illustrativeembodiment; and

FIG. 20 depicts a flowchart of an example process for fabricating aflip-chip quantum computing device in accordance with an illustrativeembodiment.

DETAILED DESCRIPTION

The illustrative embodiments used to describe the invention generallyaddress and solve the above-described problems or needs and otherrelated problems or needs by providing a fabrication method forfabricating transmon qubit flip-chip structures that are usable inflip-chip quantum computing devices. The illustrative embodiments alsoprovide a system for fabricating the transmon qubit flip-chip structuresfor flip-chip quantum computing devices.

With reference to the figures and in particular with reference to FIG.1, these figures are example diagrams of data processing environments inwhich illustrative embodiments may be implemented. FIG. 1 is only anexample and is not intended to assert or imply any limitation withregard to the environments in which different embodiments may beimplemented. A particular implementation may make many modifications tothe depicted environments based on the following description.

FIG. 1 depicts a block diagram of a network of data processing systemsin which illustrative embodiments may be implemented. Data processingenvironment 100 is a network of computers in which the illustrativeembodiments may be implemented. Data processing environment 100 includesnetwork 102. Network 102 is the medium used to provide communicationslinks between various devices and computers connected together withindata processing environment 100. Network 102 may include connections,such as wire, wireless communication links, or fiber optic cables.

Clients or servers are only example roles of certain data processingsystems connected to network 102 and are not intended to exclude otherconfigurations or roles for these data processing systems. Server 104and server 106 couple to network 102 along with storage unit 108.Software applications may execute on any computer in data processingenvironment 100. Clients 110, 112, and 114 are also coupled to network102. A data processing system, such as server 104 or 106, or client 110,112, or 114 may contain data and may have software applications orsoftware tools executing thereon.

Device 132 is an example of a mobile computing device. For example,device 132 can take the form of a smartphone, a tablet computer, alaptop computer, client 110 in a stationary or a portable form, awearable computing device, or any other suitable device. Any softwareapplication described as executing in another data processing system inFIG. 1 can be configured to execute in device 132 in a similar manner.Any data or information stored or produced in another data processingsystem in FIG. 1 can be configured to be stored or produced in device132 in a similar manner.

Application 105 implements an embodiment described herein. Fabricationsystem 107 is a software component of any suitable system forfabricating a quantum device, such as a Josephson junction, a qubit, andother superconducting structures used in quantum computing devices.Generally, fabrication systems and their corresponding softwarecomponents for manufacturing superconducting devices, including devicesfor quantum computing usage, are known. Application 105 providesinstructions to such a known fabrication system via fabricationapplication 107 for causing the assembly of a novel flip-chip quantumdevice contemplated in the illustrative embodiments, in a mannerdescribed herein.

With reference to FIG. 2, this figure depicts a qubit for use in aquantum processor. Qubit 200 includes capacitor structure 202 andJosephson junction 204. Josephson junction 204 is formed by separatingtwo thin-film superconducting metal layers by a non-superconductingmaterial. When the metal in the superconducting layers is caused tobecome superconducting—e.g. by reducing the temperature of the metal toa specified cryogenic temperature—pairs of electrons can tunnel from onesuperconducting layer through the non-superconducting layer to the othersuperconducting layer. In superconducting qubit 200, Josephson junction204—which has a small inductance—is electrically coupled in parallelwith capacitor structure 202, forming a nonlinear resonator.

With reference to FIG. 3, this figure depicts an example cross-sectionview of a flip-chip quantum computing device illustrating a problem thatcan be solved using an illustrative embodiment. Flip-chip quantumcomputing device 300 includes a J-chip 302 having a substrate 303.Substrate 303 is selected to be a suitable material on which Josephsonjunctions can be formed, and eventually the material is suitable forforming qubits using the Josephson junctions. Substrate 303 includesJosephson junction 304 formed on a first surface of substrate 303. Inthe embodiment, Josephson junction 304 has an associated impedance,which contributes in setting a qubit resonance frequency for the qubitin which Josephson junction 304 might be used, as described herein.

Substrate 303 comprises a material, which when operating in a cryogenictemperature range, exhibits a Residual Resistance Ratio (RRR) of atleast 100, and a thermal conductivity of greater than a 1 W/(cm*K) at 4Kelvin. RRR is the ratio of the resistivity of a material at roomtemperature and at 0 K. Because 0 K cannot be reached in practice, anapproximation at 4 K is used. For example, substrate 303 may be formedusing sapphire, silicon, quartz, gallium arsenide, fused silica,amorphous silicon, or diamond for operations in the temperature range of77 K to 0.01K. These examples of substrate materials are not intended tobe limiting. From this disclosure those of ordinary skill in the artwill be able to conceive of many other materials suitable for formingsubstrate 303 and the same are contemplated within the scope of theillustrative embodiments.

Flip-chip quantum computing device 300 further includes an interposerchip 306 including an interposer substrate 307. Interposer substrate 307comprises a material that exhibits an RRR of at least 100, and a thermalconductivity of greater than a 1 W/(cm*K) at 4 Kelvin. For example,interposer substrate 307 may be formed using sapphire, silicon, quartz,gallium arsenide, fused silica, amorphous silicon, or diamond foroperations in the temperature range of 77 K to 0.01K. These examples ofsubstrate materials are not intended to be limiting. From thisdisclosure those of ordinary skill in the art will be able to conceiveof many other materials suitable for forming substrate 307 and the sameare contemplated within the scope of the illustrative embodiments. In aparticular embodiment, one or more of substrate 303 and interposersubstrate 307 are formed of silicon or another suitable substratematerial.

Interposer chip 306 includes a conventional ground plane 308 formed onthe first surface of interposer substrate 307. In a particularembodiment, ground plane 308 is formed of a superconductive material,multiple superconductive materials, a metal material, or a combinationthereof.

J-chip 302 includes first landing pad 312A and second landing pad 312Bformed on the first surface of substrate 303. First landing pad 312A andsecond landing pad 312B are parts of a ground plane (not shown) onJ-chip 302. In a particular embodiment, landing pads 312A-B are formedof a superconductive material, multiple superconductive materials, ametal material, or a combination thereof.

Ground plane 308 of interposer chip 306 is bonded to J-chip 302 by firstbump bond 310A and second bump bond 310B. In some embodiments, a singlebump bond, or more than two bump bonds may also be used to bond groundplane 308 of interposer 306 with J-chip 302. In such embodiments, J-chip302 can be formed with a suitable number of landing pads to enable adesired number of ground plane bonds.

Bonding forms an electrical connection between interposer chip 306 andJ-chip 302 through first bump bond 310A and first landing pad 312A, andthrough second bump bond 310B and second landing pad 312B. In anembodiment, ground plane 308, first landing pad 312A, and second landingpad 312B are formed using at least one of Aluminum, Niobium, Titanium,Titanium Nitride, Palladium, Gold, Silver, Copper, or Platinum foroperations in the temperature range of 77 K to 0.01K. In an embodiment,bump bonds 310A, 310B are formed using Indium, Tin, and alloys ofBismuth for operations in the temperature range of 77 K to 0.01K. Theseexamples of ground plane, bump bond materials and landing pad materialsare not intended to be limiting. From this disclosure those of ordinaryskill in the art will be able to conceive of many other materialssuitable for forming the first layer and the same are contemplatedwithin the scope of the illustrative embodiments.

Qubit resonance frequency is difficult to control due to variations inJosephson junction inductance during fabrication. Josephson junctionsmade by shadow evaporation, e.g., by a Dolan bridge technique, naturallyshow variations in their Josephson inductance. For identically designedand fabricated/processed single junction transmon qubits, each qubit maynaturally have a different resonant frequency (e.g., with a variation of100 MHz-200 MHz). Such conditions may lead to frequency collisions forfixed frequency qubits using cross-resonance entangling gates such asfrequency collisions between a qubit that uses Josephson junction 304and a second, coupled qubit, which uses another Josephson junction onJ-chip 302.

The illustrative embodiments recognize that preventing frequencycollisions is a challenging issue for fixed frequency superconductingqubits, and changing or modifying the qubit frequency after chipfabrication is difficult using conventional methods. The frequency of aqubit is inversely proportional to the square root of the product of theJosephson inductance and the total capacitance across the Josephsonjunction. Accordingly, approaches to address frequency collisionsinclude changing the single-junction transmon qubit frequency bymodifying the junction inductance or the total capacitance across (e.g.,in parallel with) the junction.

Several approaches have been proposed to adjust the junction inductancein order to adjust the resonance frequency, but each have limitationsand drawbacks. For example, changing the inductance is difficult toperform precisely. Alternately, frequency adjustment can be performed bychanging capacitance, for example, by etching the substrate (e.g., asilicon (Si) substrate) in the gap of a planar capacitor to change theeffective dielectric constant. However, such etching exposes theJosephson junction to significantly more fabrication processes. Inaddition, etching and related processing can introduce additional lossmechanisms. Further, etching and related processing can typically onlybe used to decrease capacitance and increase qubit frequency, but not toincrease capacitance and correspondingly decrease qubit frequency.

An embodiment provides for a a flip-chip geometry including a J-chip andan interposer chip, which is usable to form a qubit in a quantumprocessor. The J-chip includes a plurality of Josephson junctionsdefined on a substrate.

An embodiment provides for a novel design and fabrication method of aquantum computing device in a flip-chip geometry. In the embodiment, adesign/fabrication system designs and fabricates a J-chip having aplurality of Josephson junctions using known processes for fabricating aJosephson junction. The design/fabrication system further designs andfabricates an interposer chip.

Each fabricated Josephson junction has normal-state resistance, whichcan be measured, for example, by electrically probing the Josephsonjunction resistance above the superconducting transition temperature. Aresonance frequency of a qubit that uses a particular Josephson junctioncan be predicted based upon the measured Josephson junction resistanceof that particular Josephson junction. A particular embodiment uses afitted curve relating Josephson junction resistance to frequency tocalculate the predicted frequency of such a qubit. Although variousembodiments describe measurement of a resistance of a Josephsonjunction, in other embodiments measurement of an impedance or inductanceof the Josephson junction, may be used to predict the resonancefrequency of a qubit that uses the Josephson junction.

In an embodiment, the design/fabrication system determines possiblefrequency collisions based upon the predicted resonance frequencyresulting from a set of more than one Josephson junctions fabricated onJ-chip 302. Specifically, the embodiment determines the predictedresonance frequency of a possible qubit that might use a Josephsonjunction from the set of Josephson junctions. From the set of possiblequbits, the embodiment determines a first subset of possible qubits(which use a corresponding first subset of Josephson junctions from theset of Josephson junctions), which satisfy a frequency collisionseparation threshold. Correspondingly and optionally, from the set ofpossible qubits, the embodiment determines a second subset of possiblequbits (which use a corresponding second subset of Josephson junctionsfrom the set of Josephson junctions), which fail to satisfy thefrequency collision separation threshold.

In an embodiment, where the qubit resonance frequencies are a functionof a separation gap distance between the interposer chip and the J-chip,the design/fabrication system also determines a suitable separation gapdistance between the interposer chip and the J-chip based upon thedesired frequency adjustment, frequency tuning range, and sensitivity,which would result in an acceptable numerosity of qubits in the firstsubset. In the embodiment, the design/fabrication system bonds theinterposer chip and J-chip at the separation gap distance to achieve thedesired qubit frequencies and qubit numerosity in the flip-chiparrangement.

In a particular embodiment, the design/fabrication system bonds theinterposer chip and J-chip. In one embodiment, the bonding is performedusing a bump bond process. In other particular embodiments, othersuitable methods of bonding the interposer chip and the J-chip may beused.

Another embodiment provides a fabrication method for flip-chip quantumcomputing devices, such that the method can be implemented as a softwareapplication. The application implementing a fabrication methodembodiment can be configured to operate in conjunction with an existingsuperconducting fabrication system—such as a lithography system.

For the clarity of the description, and without implying any limitationthereto, the illustrative embodiments are described using an examplenumber of Josephson junctions in a set of Josephson junctions, or anumber of qubits using a subset of the set of Josephson junctions,arranged on a substrate. An embodiment can be implemented with adifferent number of Josephson junctions in the set of Josephsonjunctions, different number of Josephson junctions in the subset to formqubits, different arrangements, a superconducting device other than aqubit formed using a Josephson junction in the subset, types of quantumcomputing devices not based on cryogenic superconductors, or somecombination thereof, within the scope of the illustrative embodiments.

Furthermore, a simplified diagram of the example flip-chip geometry isused in the figures and the illustrative embodiments. In an actualfabrication of a flip-chip, additional structures that are not shown ordescribed herein, or structures different from those shown and describedherein, may be present without departing the scope of the illustrativeembodiments. Similarly, within the scope of the illustrativeembodiments, a shown or described structure in the example flip-chip maybe fabricated differently to yield a similar operation or result asdescribed herein.

Differently shaded portions in the two-dimensional drawing of theexample structures, layers, and formations are intended to representdifferent structures, layers, materials, and formations in the examplefabrication, as described herein. The different structures, layers,materials, and formations may be fabricated using suitable materialsthat are known to those of ordinary skill in the art.

A specific shape, location, position, or dimension of a shape depictedherein is not intended to be limiting on the illustrative embodimentsunless such a characteristic is expressly described as a feature of anembodiment. The shape, location, position, dimension, numerosity, orsome combination thereof, are chosen only for the clarity of thedrawings and the description and may have been exaggerated, minimized,or otherwise changed from actual shape, location, position, or dimensionthat might be used in actual lithography to achieve an objectiveaccording to the illustrative embodiments.

Furthermore, the illustrative embodiments are described with respect toa specific actual or hypothetical superconducting device, e.g., a qubitthat is presently viable, only as an example. The steps described by thevarious illustrative embodiments can be adapted for fabricating avariety of quantum computing devices in a similar manner, and suchadaptations are contemplated within the scope of the illustrativeembodiments.

An embodiment when implemented in an application causes a fabricationprocess to perform certain steps as described herein. The steps of thefabrication process are depicted in the several figures. Not all stepsmay be necessary in a particular fabrication process. Some fabricationprocesses may implement the steps in different order, combine certainsteps, remove or replace certain steps, or perform some combination ofthese and other manipulations of steps, without departing the scope ofthe illustrative embodiments.

The illustrative embodiments are described with respect to certain typesof materials, electrical properties, thermal properties, structures,formations, shapes, layers orientations, directions, steps, operations,planes, dimensions, numerosity, data processing systems, environments,components, and applications only as examples. Any specificmanifestations of these and other similar artifacts are not intended tobe limiting to the invention. Any suitable manifestation of these andother similar artifacts can be selected within the scope of theillustrative embodiments.

The illustrative embodiments are described using specific designs,architectures, layouts, schematics, and tools only as examples and arenot limiting to the illustrative embodiments. The illustrativeembodiments may be used in conjunction with other comparable orsimilarly purposed designs, architectures, layouts, schematics, andtools.

An advantage that may be provided by an embodiment is that there is noneed for additional processes on the J-chip after fabrication whichprovides for no risk of junction damage or failure.

The examples in this disclosure are used only for the clarity of thedescription and are not limiting to the illustrative embodiments. Anyadvantages listed herein are only examples and are not intended to belimiting to the illustrative embodiments. Additional or differentadvantages may be realized by specific illustrative embodiments.Furthermore, a particular illustrative embodiment may have some, all, ornone of the advantages listed above.

With reference to FIG. 4, this figure depicts a block diagram of anexample J-chip configuration in accordance with an illustrativeembodiment. Application 105 in FIG. 1 interacts with fabrication system107 to produce or manipulate configuration 400 as described herein.Substrate 402 is an example of substrate 302 in FIG. 3.

An embodiment causes the fabrication system to deposit material 404,thus forming a set of pads 408. For example, a mask can be designed toinclude the layout of one or more pads 408. The fabrication systemoperating in conjunction with an embodiment uses the mask to patternmaterial 404 in the shape of pads 408 on (or in) substrate 402 via thephotolithographic process described earlier. A pattern corresponding topads 408 etched in a hard mask layer can also allow a photolithographicprocess to deposit material 404 in the shape of pads 408. These andother possible manners of forming pads 408 via lithographic processesare contemplated within the scope of the illustrative embodiments.

Set of pads 408 comprises material 404 with high electrical and thermalconductivity (above a threshold RRR and above a threshold thermalconductivity) in the cryogenic temperature range. In an embodiment, setof pads 408 are formed using at least one of Aluminum, Niobium,Titanium, Titanium Nitride, Palladium, Gold, Silver, Copper, or Platinumfor operations in the temperature range of 77 K to 0.01K. These examplesof layer materials are not intended to be limiting. From thisdisclosure, those of ordinary skill in the art will be able to conceiveof many other materials suitable for forming the set of pads and thesame are contemplated within the scope of the illustrative embodiments.

In an embodiment, set of pads 408 is deposited on one side, e.g. a sideof substrate 402 that will face the interposer in the flip-chipconfiguration. For example, set of pads 408 can be a thin filmdeposition of particles 406 on substrate 402. Particles 406 can bedeposited using a thin film deposition technique in lithography. Thisexample of a deposition method is not intended to be limiting. From thisdisclosure, those of ordinary skill in the art will be able to conceiveof many other methods and process suitable for forming the set of padsand the same are contemplated within the scope of the illustrativeembodiments. In an embodiment, particles 406 are of a material that isusable for electrically isolating an Under Bump Metal (UBM) layer(described at least in FIG. 5) from substrate 402. In one embodiment,pads 408 are optional, e.g., when the substrate or an underlyingstructure, e.g., a ground plane formed in some other manner, has arequired electrical characteristic for fabricating a UBM layer atop suchstructure, the UBM layer can be formed to a desired height directly onthe substrate or the underlying structure.

With reference to FIG. 5, this figure depicts a block diagram of anexample configuration reached in the fabrication of the flip-chip devicein accordance with an illustrative embodiment. Application 105 in FIG. 1interacts with fabrication system 107 to produce or manipulateconfiguration 500 as described herein. Substrate 502 is an example ofsubstrate 402 in FIG. 4, or substrate 302 in FIG. 3. Pads 504 areexamples of pads 408 from FIG. 4, and may be optional for the reasonsdescribed earlier.

A UBM is a conductive structure which is electrically coupled to aJosephson junction on the J-chip. The material of the UBM allowsreliable formation and adhesion of a bump of a suitable conductivematerial, e.g., a cryogenic superconducting solder bump. Thus,functionally, the UBM operates as a part of the superconducting pathwayfrom a bump to the Josephson junction in cryogenic operating conditions.

Configuration 500 is a configuration that is optionally reachable fromconfiguration 400 in FIG. 4 using a suitably configured mask in alithographic process. Alternatively, when pads 408 of FIG. 4 are notutilized, configuration 500 is reachable from configuration 300 in FIG.3, and the UBM structures described in this figure can be formed up to adesired height directly on a substrate or another structure, in a mannerdescribed earlier herein, using a suitably configured mask in alithographic process.

The depiction of the mask and the description of the lithographytechniques should not be construed as limiting on the manner of formingthe structures described herein. The depicted mask and depicted mannerof depositing the material are merely simplified and generalizedexamples. Lithography of the depicted structures is possible in manyways. For example, lithography of the described structures is presentlyaccomplished by patterning a resist with photolithography (light) orebeam lithography (electron beam), developing the resist, then eithersubtracting deposited material from the openings in the resist, ordepositing material in the openings in the resist. The resist is removedat the end. Pads, resonators and ground planes are usually made bysubtraction, and junctions and UBM are usually made by addition (andsubsequent lift off process) using the presently available fabricationfacilities. Fabrication processes and technology is constantly changingand other methods of forming the described structures are within thecontemplations of the illustrative embodiments so long as the resultingstructures have the electrical, mechanical, thermal, and operatingcharacteristics as described herein.

In one embodiment, first layer 510 is patterned using a mask on pad 504via a lithographic process, e.g., using a depositing method, to form aUBM. In another embodiment, as depicted in configuration 550, firstlayer 552 is patterned up to a desired height on substrate 502 via alithographic process to form a UBM in a manner described herein. As anon-limiting example, first layer 510 can be patterned using a thin filmdeposition technique in lithography to deposit particles 508. As anotherexample, first layer 510 can be patterned using a sputtering techniqueknown in lithography. These examples of methods of forming the UBM arenot intended to be limiting. From this disclosure, those of ordinaryskill in the art will be able to conceive of many other methods andprocess suitable for forming the UBM and the same are contemplatedwithin the scope of the illustrative embodiments. Further descriptionusing UBM 510 of configuration 500 is only for the clarity of thedescription and not to imply any limitation on any embodiments. Aconfiguration shown using UBM 510 or an equivalent thereof can beimplemented using UBM 552 or an equivalent thereof, without departingthe scope of the illustrative embodiments.

With reference to FIG. 6, this figure depicts a block diagram of anexample configuration reached in the fabrication of the flip-chip devicein accordance with an illustrative embodiment. Application 105 in FIG. 1interacts with fabrication system 107 to produce or manipulateconfiguration 600 as described herein. Substrate 602 is an example ofsubstrate 502 in configuration 500 or 550 of FIG. 5. Pads 604 are UBMs,configured in the manner of either a combination of structure 504 andfirst layer 510, or in the manner of layer 552 in FIG. 5. An embodimentcauses a fabrication system, which is configured to fabricate quantumcomputing devices or components therefor, as described with respect toFIG. 1, to pattern material 606 into Josephson junction 610 on J-chipsubstrate 602. As one non-limiting example, Josephson junction 610 canbe patterned using a suitably designed mask with photolithography.

With reference to FIG. 7, this figure depicts an example graph forcalculating a predicted frequency of a qubit based upon a measuredjunction resistance in accordance with an illustrative embodiment. FIG.7 illustrates a graph of a predicted qubit frequency f01 versus aJosephson junction resistance R. Graph 700 includes curve 702 and curve704. In accordance with an embodiment, a resistance of the Josephsonjunction is measured (e.g., by electrical probing) to obtain aresistance R. Based upon the measured resistance R, a predictedresonance frequency of the qubit that uses that Josephson junction maybe determined by reading the corresponding value on the Y-axis of curves702 and 704 in graph 700.

With reference to FIG. 8, this figure depicts a block diagram of anexample configuration reached in the fabrication of the flip-chip devicein accordance with an illustrative embodiment. Application 105 in FIG. 1interacts with fabrication system 107 to produce or manipulateconfiguration 800 as described herein. Configuration 800 includesinterposer substrate 802, a set of pads 804 formed on a frontside of theinterposer substrate 802, and a set of resonator signal lines 806. Inone embodiment, pads 804 and resonator signal lines 806 are formed of amaterial that exhibits similar electro-thermal characteristics as thematerial of ground plane 308 in FIG. 3. In another embodiment, pads 804and resonator signal lines 806 are formed of the same material as groundplane 308 in FIG. 3. In one embodiment, pads 804 and resonator signallines 806 are patterned at the same lithographic step in which groundplane 308 is patterned in FIG. 3. In another embodiment, pads 804 andresonator signal lines 806 are patterned separately and after thelithographic step in which ground plane 308 is patterned in FIG. 3.

An embodiment causes the fabrication system to deposit material 810,patterned for example with a deposition process 808 in photolithography,thus forming a first layer 812 on the set of pads 804. In an embodiment,first layer 812 is patterned using materials and photolithographicprocesses functionally similar to those used for patterning first layer510 on pads 504 in FIG. 5. First layer 812 forms a UBM layer over acorresponding pad 804.

With reference to FIG. 9, this figure depicts a block diagram of anexample configuration reached in the fabrication of the flip-chip devicein accordance with an illustrative embodiment. Application 105 in FIG. 1interacts with fabrication system 107 to produce or manipulateconfiguration 900 as described herein. Configuration 900 is furtherdevelopment of configuration 800 in FIG. 8, and reference numerals 802,804, 806, and 812 are indicative of the same or similar structures asdepicted and described with respect to FIG. 8.

An embodiment causes the fabrication system perform a suitablelithography operation 910 to deposit material 912, thus forming a secondlayer 914 on the first layer 812. In an embodiment, second layer 914 isdeposited on first layer 812 only on a subset of the set of pads-UBMstructures 804-812. In an embodiment, the subset of the set of pads-UBMstructures 804-812 corresponds to a selected subset of the set ofJosephson junction. As described herein, not all Josephson junctions ina set of Josephson junctions may satisfy the requirements for formingqubits, and only a subset of Josephson junctions might be selected. Thesubset of the set of pads-UBM structures 804-812 that correspond to theselected subset of Josephson junctions are the ones that receive secondlayer 914. In practice, an embodiment causes a lithography mask to becreated such that only the selected subset of pads-UBM structures804-812 receive the deposit of material 912 to form second layer 914.Other methods, such as, but not limited to, hard mask creation insteadof lithographic mask, are also possible for a similar purpose and thesame are contemplated within the scope of the illustrative embodiments.

In an embodiment, second layer 914 is a set of solder bumps. In anembodiment, an instance of second layer 914 is a bump formed usingIndium, Tin, and Bismuth, or some combination thereof, for operations inthe temperature range of 77 K to 0.01K. These examples of a second layermaterial are not intended to be limiting. From this disclosure those ofordinary skill in the art will be able to conceive of many othermaterials suitable for forming the second layer and the same arecontemplated within the scope of the illustrative embodiments.

In an embodiment, second layer 914 is deposited on the first layer 906.For example, second layer 914 is an injection molded soldering (IMS)deposition of particles 912 onto first layer 812. In someimplementations, layer 812 may be absent and particles 912 may bedeposited onto pad 804 to form bump 914. In some other implementations,pad 804 may be absent and bump 914 may be formed on UBM layer 812. Insome other implementations, the pad-UBM layer combination 804-812 may beformed in an alternative manner using alternative materials but for asimilar purpose—to enable electrical connectivity to a Josephsonjunction from a point in the interposer chip. In such a case, bump 914may be formed at or over the alternative structure without departing thescope of the illustrative embodiments.

With reference to FIG. 10, this figure depicts a block diagram of anexample flip chip configuration reached in the fabrication of theflip-chip device in accordance with an illustrative embodiment.Application 105 in FIG. 1 interacts with fabrication system 107 toproduce or manipulate configuration 1000 as described herein.

An embodiment causes the fabrication system to cause interposer chip 802with its corresponding structures to be oriented relative to J-chip 602with its corresponding structures to be oriented relative to one anothersuch that their respective structures face each other to form theflip-chip configuration. For example, the interposer chip is shown asflipped over the J-chip such that bumps 914 described in FIG. 9 makephysical and electrical contact with structures 604 on the J-chip 602 ofFIG. 6.

With reference to FIG. 11, this figure depicts a top-down schematic viewof an example J-chip configuration in accordance with an illustrativeembodiment. Application 105 in FIG. 1 interacts with fabrication system107 to produce or manipulate configuration 1100 to form configuration1101 as described herein. In configuration 1100 assume that J-chipsubstrate 602 of FIG. 6 is configured with three non-limiting exampleJosephson junctions 610. Each Josephson junction 610 is electricallycoupled with a corresponding pair of pads 504, and the pairs of pads 504have UBM layers 510 fabricated in the manner described in FIG. 5.

Only three Josephson junctions and their corresponding pairs ofconnection structures are shown in the interest of clarity and not toimply any limitation on the illustrative embodiments to configurationsof only three or less Josephson junctions on a contemplated J-chip. Anembodiment can be implemented and practiced with a J-chip comprising anynumber of Josephson junctions and their corresponding connectionstructures without any limitation imposed by the illustrativeembodiments, and limited only by the state of the art in the field atany given time.

In an embodiment, application 105 determines a subset of the set ofJosephson junctions 610 are to be removed to avoid frequency collision.An embodiment causes the fabrication system to disable the subset of theset of Josephson junctions 610, thus forming configuration 1101, adisabled Josephson junction is represented in configuration 1101 asJosephson junction 1112. A Josephson junction can be disabled in avariety of ways, including but not limited to physically destroying oraltering the Josephson junction, electrically destroying or altering anelectrical characteristic of the Josephson junction, disconnecting theJosephson junction from the Josephson junction's corresponding one orboth pads, physically destroying or altering the Josephson junction'sone or both pads, electrically destroying or altering an electricalcharacteristic of the Josephson junction's one or both pads, physicallydestroying or altering a UBM layer on one or both pads of the Josephsonjunction, electrically destroying or altering an electricalcharacteristic of the UBM layer on one or both pads of the Josephsonjunction, covering up one or both pads or one or both UBMs of theJosephson junction by fabricating an insulating layer, or somecombination of these and many other possible ways of rendering aJosephson junction unusable.

For example, in an embodiment, the fabrication system ablates the subsetof the set of Josephson Junctions 1112 from the surface of J-chipsubstrate 602. For example, the fabrication system can use laserablation to remove the subset of the set of Josephson Junctions 1112. Asanother example, the fabrication system can use focused ion beam (FIB)to remove the subset of the set of Josephson Junctions 1112. In anotherembodiment, the fabrication system destroys the subset of the set ofJosephson Junctions 1112 by disconnecting electrical connectors 1108 and1110 coupling Josephson junction 1112 to its corresponding pair of pads504. The disconnecting of connectors 1108 and 1110 can also be performedvia ablation, FIB, or another suitable method.

With reference to FIG. 12, this figure depicts a block diagram of analternative example configuration in accordance with an illustrativeembodiment. Application 105 in FIG. 1 interacts with fabrication system107 to produce or manipulate configuration 1200 as described herein.Configuration 1200 is an example of configuration 800 in FIG. 8.Configuration 1200 starts with interposer substrate 802, a set of pads804, a set of resonators 806, and a first layer 812 formed on the set ofpads 804.

A Josephson junction can be disabled or rendered unavailable for use ina quantum computing apparatus by simply not connecting to that Josephsonjunction. For example, an undesirable Josephson junction—along with theJosephson junction's connected pads and UBM layers—can be leftunconnected by simply not forming a bump at the location on theinterposer chip, where the location corresponds to the UBM layers of theJosephson junction. Absent a bump, a pad on the interposer at thelocation will not make electrical contact with the UBM layer of theJosephson junction, rendering the Josephson junction unusable.

An embodiment causes a mask to be constructed that certain interposerpads are blocked from receiving material 1212 deposited using process1210 in the fabrication system. Thus, as can be seen in the exampleresult of such selective depositing, second layer 904 is formed only onsome pad-UBM combination 804-812 and not others. For example, a singlepad-UBM combination 804-812 in area marked 1202 is devoid of the secondlayer—the bump. As another example, a pair of pad-UBM combination804-812 in area marked 1204 is devoid of the bump.

With reference to FIG. 13, this figure depicts a block diagram of anexample J-chip assembly in accordance with an illustrative embodiment.Assembly 1300 shows, as a non-limiting example, an interposer chipflipped over a J-chip in which some Josephson junctions from the J-chipare connected to circuits and components on the interposer chip andothers have been at least left disconnected, and preferably renderedunusable.

With reference to FIG. 14, this figure depicts a block diagram of anexample detachable J-chip configuration in accordance with anillustrative embodiment. Application 105 in FIG. 1 interacts withfabrication system 107 to produce or manipulate configuration 1400 asdescribed herein. Configuration 1400 is an example of configuration 500in FIG. 5.

Configuration 1400 comprises J-chip substrate 1402, similar to substrate502 in FIG. 5. Pads 1404 are similar to pads 504 in FIG. 5. First layers1406 are similar to first layers 510 in FIG. 5.

An embodiment causes a fabrication system, such as fabrication system107 in FIG. 1, to create a set of protrusions 1412 on a first layer 1406of a set of pads 1404 of substrate 1402. For example, an embodiment cancause mask 1408 in the fabrication system to deposit material 1410, thusforming the set of protrusions 1412. In an embodiment, fabricationsystem 107 comprises a wire bonder to deposit material 1410 and formprotrusion 1412. For example, the wire bonder can form a first half of aball bond before pulling upwards to deposit the remainder of theprotrusion. In an embodiment, protrusion 1412 is a column. For example,protrusion 1412 can have a conical, triangular, cylindrical, orrectangular cross-section.

In an embodiment, protrusion 1412 comprises a material 1410 with apredetermined ductility (above a threshold) at a room temperature range.In an embodiment, protrusion 1412 is formed using a material thatexhibits an elongation at break of at least twenty percent at a roomtemperature range. For example, protrusion 1412 may be formed usinggold, platinum, or a gold-coated superconducting material. Theseexamples of protrusion material, qubit substrate material, protrusionshapes, and deposition methods are not intended to be limiting. Fromthis disclosure those of ordinary skill in the art will be able toconceive of many other materials and methods suitable for forming thesubstrate, J-chip, and protrusions and the same are contemplated withinthe scope of the illustrative embodiments.

With reference to FIG. 15, this figure depicts a block diagram of anexample detachable configuration in accordance with an illustrativeembodiment. Application 105 in FIG. 1 interacts with fabrication system107 to produce or manipulate configuration 1500 as described herein.

Configuration 1500 comprises an interposer chip configuration built onsubstrate 802 in the manner of FIG. 9. configuration 1550 furthercomprises a J-chip configuration built on substrate 602 in the manner ofFIG. 6 and further transformed in the manner of FIG. 14.

An embodiment causes the fabrication system to couple the j-chipconfiguration with the interposer chip configuration such that aprotrusion 1412 on the J-chip detachably but conductively couples with acorresponding bump 914 on the interposer chip. Note that a protrusion1412 may be formed and interfaced with a bump on the interposer evenwhen a corresponding Josephson junction has been disabled in a mannerdescribed herein.

In one embodiment, the detachable conductive coupling between protrusion1412 and bump 914 is achieved by causing fabrication system 107 to coldweld protrusion 1412 with a solder bump 914. For example, protrusion1412 pierces the corresponding solder bump 914. Cold welding is awelding process in which coupling takes place at the interface of thetwo parts to be welded, wherein the interface is at a room temperaturerange. In cold welding, the interface is in a solid state. In thismanner, a set of protrusion detachably but electrically conductivelycouples to a corresponding set of bumps.

With reference to FIG. 16, this figure depicts a block diagram of anexample detachable conductive coupling configuration in accordance withan illustrative embodiment. Configuration 1600 is an example of thecold-welded connection between the set of protrusions and the set ofsolder bumps in FIG. 15. Configuration 1600 comprises pad 804 and UBMlayer 812 on an interposer chip as in FIG. 8, bump 914 as in FIG. 9, pad504 and UBM layer 510 on a J-chip as in FIG. 5, and protrusion 1412 asin FIG. 14.

In an embodiment, bump 914 comprises a material with a predeterminedductility (above a threshold) at a room temperature range. In anembodiment, bump 914 is formed using a material that exhibits anelongation at break of at least twenty percent at a room temperaturerange. For example, bump 914 is formed using at least one of Indium,Tin, Lead, Bismuth, and any combination thereof. In an embodiment, bump914 comprises a material which exhibits superconductivity in thecryogenic temperature range. In one embodiment, bump 914 contacts UBMlayers on the interposer chip as well as on the J-chip. In other words,bump 914 extends fully—and provides a complete electrically conductivepath between UBM layers 812 and 510 as shown.

With reference to FIG. 17, this figure depicts a block diagram ofanother example configuration of a detachable conductive coupling inaccordance with an illustrative embodiment. Configuration 1700 is anexample of the cold-welded connection between the set of protrusions andthe set of solder bumps in FIG. 15. Configuration 1700 pad 804 and UBMlayer 812 on an interposer chip as in FIG. 8, bump 914 as in FIG. 9, pad504 and UBM layer 510 on a J-chip as in FIG. 5, and protrusion 1412 asin FIG. 14.

In an embodiment, bump 914 comprises a material with a predeterminedductility (above a threshold) at a room temperature range. In anembodiment, bump 914 is formed using a material described with respectto FIG. 16. In one embodiment, bump 914 contacts the UBM layer on onlyone chip but not the other. For example, as shown, bump 914 contacts UBMlayer 812 on the interposer chip but not UBM layer 510 on the J-chip. Inother words, bump 914 extends partially—and provides a completeelectrically conductive path between UBM layers 812 and 510 only whenpierced by protrusion 1412, as shown. In an embodiment, a capacitance ofthe electrical connection is determined by a distance between the firstpad 804 and the second pad 504. For example, the capacitance isinversely proportional to a distance, or gap height, between the firstpad 804 and the second pad 504. In an embodiment, protrusion 1412 has aheight corresponding to a desired capacitance of the electricalconnection. In an embodiment, the gap height is a function of the heightof the protrusion 1412 and the compression force during cold welding.For example, the gap height can have an inverse relationship with theamount of the compression force during cold welding. As another example,the gap height can have a direct relationship with the height of theprotrusion 1412.

These examples of substrate materials, bump materials, depositionmethods, and pad materials are not intended to be limiting. From thisdisclosure those of ordinary skill in the art will be able to conceiveof many other materials and deposition methods suitable for forming thecomponents of the device and the same are contemplated within the scopeof the illustrative embodiments. In an embodiment, a height ofcorresponding protrusions differs between a set of protrusions formed ona surface. For example, a height of protrusions can differ toaccommodate warpage of a substrate.

With reference to FIG. 18, this figure depicts a flowchart of an exampleprocess for fabricating a flip-chip quantum computing device inaccordance with an illustrative embodiment. In one or more embodiments,process 1800 is implemented in application 105, which causes afabrication system, such as fabrication system 107 in FIG. 1 to performthe operations described herein.

In block 1802, the application causes the fabrication system to form afirst set of pads on a J-chip. In block 1804, the application causes thefabrication system to deposit a first layer on the first set of pads. Inblock 1806, the application causes the fabrication system to form a setof Josephson junctions on the J-chip, each Josephson junction in the setof Josephson junctions having a junction resistance (slightly inductiveimpedance).

In block 1808, the application causes the fabrication system to measurea Josephson junction resistance of each of the Josephson junctions in aset of Josephson junctions formed in block 1806, for example, byelectrically probing the Josephson junction resistance of each Josephsonjunction. The application causes the fabrication system to calculate apredicted frequency of each qubit that could be based upon a particularJosephson junction having a measured Josephson junction resistance. In aparticular embodiment, the design/fabrication system uses a fitted curverelating Josephson junction resistance to frequency to calculate thepredicted frequency of each qubit, such as graph 700 of FIG. 7.

In block 1810, the application causes the fabrication system to select,in response to the calculation, a first subset of the set of Josephsonjunctions to avoid or mitigate possible frequency collision betweenqubits that could result from the Josephson junctions in the firstsubset. In one embodiment, at block 1812, the fabrication systemdeposits, in response to the selection, a second layer on a secondsubset of a second set of pads on an interposer chip. In an embodiment,the second subset of the second set of pads corresponds to the selectedfirst subset of the set of Josephson junctions. In practice, theapplication causes a mask to be constructed that allows only the pads inthe second subset to receive a deposited material of the second layer.The application then causes a lithography component of the fabricationsystem to use the mask in depositing the material of the second layeronto the second subset of pads.

In an embodiment, the application causes the fabrication system todetermine a separation gap distance between the interposer chip and theJ-chip based upon the selected subset of the set of Josephson junctions,frequency tuning range, and sensitivity. In block 1814, the applicationcauses the fabrication system to bond the interposer chip and J-chip atthe determined separation gap distance to achieve the desired qubitfrequencies in the flip-chip arrangement. In a particular embodiment,the fabrication system bonds the interposer chip and J-chip using a bumpbond process. In other particular embodiments, other suitable methods ofpermanent or temporary (detachable) bonding the interposer chip and theJ-chip may be used. Process 1800 then ends.

With reference to FIG. 19, this figure depicts a flowchart of an exampleprocess for fabricating a flip-chip quantum computing device inaccordance with an illustrative embodiment. In one or more embodiments,process 1800 is implemented in application 105, which causes afabrication system, such as fabrication system 107 in FIG. 1 to performthe operations described herein.

In block 1902, the application causes the fabrication system to form afirst set of pads on a J-chip. In block 1904, the application causes thefabrication system to deposit a first layer on the first set of pads. Inblock 1906, the application causes the fabrication system to form a setof Josephson junctions on the J-chip, each Josephson junction in the setof Josephson junctions having a junction resistance.

In block 1908, the application causes the fabrication system to measurea Josephson junction resistance of each of the Josephson junctions, forexample, by electrically probing the Josephson junction resistance of aJosephson junction. The application causes the fabrication system tocalculate a predicted frequency of each qubit based upon the measuredJosephson junction resistance. In a particular embodiment, thedesign/fabrication system uses a fitted curve relating Josephsonjunction resistance of a particular Josephson junction to frequency, tocalculate the predicted frequency of a qubit that would be formed usingthat Josephson junction, such as graph 700 of FIG. 7.

In block 1910, the application causes the fabrication system to select,in response to the calculation, a first subset of the set of Josephsonjunctions to avoid possible frequency collision in the resulting qubits.In block 1912, in response to the selection, the application causes thefabrication system to removes, disables, other otherwise rendersunreachable or unusable, a second subset of the set of Josephsonjunctions. In an embodiment, an intersection of the first subset and thesecond subset is an empty set. In block 1914, the application causes thefabrication system to deposit a second layer on a second set of pads onan interposer chip. In practice, the application causes a mask to beconstructed that allows the pads in the second set to receive adeposited material of the second layer. The application then causes alithography component of the fabrication system to use the mask indepositing the material of the second layer onto the second set of pads.

In an embodiment, the application causes the fabrication system todetermine a separation gap distance between the interposer chip and theJ-chip based upon the selected subset of the set of Josephson junctions,frequency tuning range, and sensitivity. In block 1916, the fabricationsystem bonds the interposer chip and J-chip at the determined separationgap distance to achieve the desired qubit frequencies in the flip-chiparrangement. In a particular embodiment, the application causes thefabrication system to bond the interposer chip and J-chip using a bumpbond process. In other particular embodiments, other suitable methods ofbonding the interposer chip and the J-chip may be used. Process 1900then ends.

With reference to FIG. 20, this figure depicts a flowchart of an exampleprocess for fabricating a flip-chip quantum computing device inaccordance with an illustrative embodiment. In one or more embodiments,process 1800 is implemented in application 105, which causes afabrication system, such as fabrication system 107 in FIG. 1 to performthe operations described herein.

In block 2002, the application causes the fabrication system to form afirst set of pads on a J-chip. In block 2004, the application causes thefabrication system to deposit a first layer on the first set of pads. Inblock 2006, the application causes the fabrication system to deposit afirst set of stud bumps (protrusions) on the first layer of the J-chip.In block 2008, the application causes the fabrication system to a set ofJosephson junctions on the J-chip, each Josephson junction in the set ofJosephson junctions having a junction resistance.

In block 2010, the application causes the fabrication system to measurea Josephson junction resistance of each of the Josephson junctions, forexample, by electrically probing the Josephson junction resistance. Theapplication causes the fabrication system to calculate a predictedfrequency of each qubit that would result from a Josephson junction inthe set, based upon the measured Josephson junction resistance. In aparticular embodiment, the design/fabrication system uses a fitted curverelating Josephson junction resistance to frequency to calculate thepredicted frequency of each qubit, such as graph 700 of FIG. 7.

In block 2012, the application causes the fabrication system to select,in response to the calculation, a first subset of the set of Josephsonjunctions to avoid or mitigate possible frequency collision in thequbits resulting from the Josephson junctions in the first subset. Inblock 2014, in response to the selection, the application causes thefabrication system to remove, disable, or otherwise render unreachableor unusable, a second subset of the set of Josephson junctions. In anembodiment, an intersection of the first subset and the second subset isan empty set. In block 2016, the application causes the fabricationsystem to deposit, in response to the selection, a second layer on asecond set of pads on an interposer chip. In an embodiment, the secondsubset of the second set of pads corresponds to the selected firstsubset of the set of qubits. In an embodiment, the fabrication systemdeposits a second layer on the second set of pads on the interposerchip.

In block 2018, the application causes the fabrication system to form atemporary bond between the interposer chip and the J-chip. In block2020, the application measures actual frequencies of the set of qubitsresulting from the first subset of Josephson junctions, during cryogenicoperation. In block 2022, the application determines whether the actualmeasured frequencies meet a threshold for avoidance of frequencycollision.

In block 2026 (“NO” path of block 2022), the fabrication system altersfrequencies of one or more qubits in the set of qubits. In oneembodiment, fabrication system warms the flip-chip assembly, detachesthe interposer chip from the J-chip, performs a laser annealing processto alter the resistance of one or more Josephson junctions in the firstsubset of Josephson junctions. The process then temporarily bonds theinterposer chip with the J-chip in the manner of block 2018 and returnsto block 2020.

In block 2024 (“YES” path of block 2022), the application causes thefabrication system to durably bond the interposer chip and J-chip at adetermined separation gap distance to achieve the desired qubitfrequencies in the flip-chip arrangement. In a particular embodiment,the application causes the fabrication system to durably or permanentlybond the interposer chip and J-chip using a bump bond process. In otherparticular embodiments, other suitable methods of bonding the interposerchip and the J-chip may be used. In an embodiment, the applicationcauses the fabrication system to determine a separation gap distancebetween the interposer chip and the J-chip based upon the selectedsubset of the set of Josephson junctions, frequency tuning range, andsensitivity. Process 2000 then ends.

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of this invention. Althoughvarious connections and positional relationships (e.g., top, bottom,over, below, adjacent, etc.) are set forth between elements in thefollowing description and in the drawings, persons skilled in the artwill recognize that many of the positional relationships describedherein are orientation-independent when the described functionality ismaintained even though the orientation is changed. These connectionsand/or positional relationships, unless specified otherwise, can bedirect or indirect, and the present invention is not intended to belimiting in this respect. Accordingly, a coupling of entities can referto either a direct or an indirect coupling, and a positionalrelationship between entities can be a direct or indirect positionalrelationship. As an example of an indirect positional relationship,references in the present description to forming layer “A” over layer“B” include situations in which one or more intermediate layers (e.g.,layer “C”) is between layer “A” and layer “B” as long as the relevantcharacteristics and functionalities of layer “A” and layer “B” are notsubstantially changed by the intermediate layer(s).

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

Additionally, the term “illustrative” is used herein to mean “serving asan example, instance or illustration.” Any embodiment or designdescribed herein as “illustrative” is not necessarily to be construed aspreferred or advantageous over other embodiments or designs. The terms“at least one” and “one or more” are understood to include any integernumber greater than or equal to one, i.e. one, two, three, four, etc.The terms “a plurality” are understood to include any integer numbergreater than or equal to two, i.e. two, three, four, five, etc. The term“connection” can include an indirect “connection” and a direct“connection.”

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may or may not include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about”can include a range of ±8% or 5%, or 2% of a given value.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. A computer-implemented method to fabricate aquantum computing device, the method comprising: forming a first chiphaving a first substrate, a first set of pads, and a set of Josephsonjunctions disposed on the first substrate; and forming a second chiphaving a second substrate, a second set of pads disposed on the secondsubstrate opposite the first set of pads, wherein the subset of thesecond set of pads corresponds to a subset of the set of Josephsonjunctions selected to avoid frequency collision between qubits in a setof qubits, a qubit in the set of qubits resulting from a Josephsonjunction in the subset of Josephson junctions.
 2. Thecomputer-implemented method of claim 1, further comprising: forming anunusable Josephson junction in the set of Josephson junctions, wherein afirst Josephson junction in the set of Josephson junctions is modifiedto become the unusable Josephson junction responsive to the firstJosephson junction being excluded from the subset of the set ofJosephson junctions.
 3. The computer-implemented method of claim 2,further comprising: forming a disconnected pad in the first set of pads,wherein the first set of pads comprises a first pad electrically coupledto the first Josephson junction, and wherein the first pad iselectrically disconnected from the first Josephson junction to form thedisconnected pad, the disconnected pad rendering the first Josephsonjunction unusable.
 4. The computer-implemented method of claim 2,wherein an electrical property of the first Josephson junction ismodified such that the first Josephson junction no longer operates as aJosephson junction.
 5. The computer-implemented method of claim 1,wherein the subset is selected based upon a measurement of a parameterassociated with each of the set of Josephson junctions.
 6. Thecomputer-implemented method of claim 5, wherein the resonance frequencyassociated with a particular qubit is one member selected from a set of(i) a predicted resonance frequency calculated based upon the measuredparameter, and (ii) an actual measured resonance frequency of theparticular qubit.
 7. The computer-implemented method of claim 5, whereinthe parameter includes a resistance associated with a Josephson junctionin the set of Josephson junctions.
 8. The computer-implemented method ofclaim 7, wherein the resistance is a normal-state resistance of theJosephson junction.
 9. The computer-implemented method of claim 1,further comprising: forming a first set of protrusions formed on thefirst chip; and forming a set of bumps formed on the first layer of thesecond chip, the set of bumps formed of a material having above athreshold ductility at a room temperature range, wherein set of bumpsare configured to cold weld to the first set of protrusions.
 10. Thecomputer-implemented method of claim 9, wherein the first set ofprotrusions is of at least one member selected from a set comprisingGold and Platinum.
 11. The computer-implemented method of claim 9,wherein the set of bumps is of at least one member selected from a setcomprising Indium, Tin, Lead, and Bismuth.
 12. The computer-implementedmethod of claim 9, further comprising: forming a flip-chip assemblycomprising the first chip detachably attached to the second chip usingthe cold weld, wherein a parameter of a Josephson junction inside theflip-chip assembly is tunable by disassembling the flip-chip assembly atthe cold weld.
 13. A superconductor fabrication system comprising alithography component, the superconductor fabrication system whenoperated on at least one die to fabricate a quantum computing deviceperforming operations comprising: forming a first chip having a firstsubstrate, a first set of pads, and a set of Josephson junctionsdisposed on the first substrate; and forming a second chip having asecond substrate, a second set of pads disposed on the second substrateopposite the first set of pads, wherein the subset of the second set ofpads corresponds to a subset of the set of Josephson junctions selectedto avoid frequency collision between qubits in a set of qubits, a qubitin the set of qubits resulting from a Josephson junction in the subsetof Josephson junctions.
 14. The superconductor fabrication system ofclaim 13, further comprising: forming an unusable Josephson junction inthe set of Josephson junctions, wherein a first Josephson junction inthe set of Josephson junctions is modified to become the unusableJosephson junction responsive to the first Josephson junction beingexcluded from the subset of the set of Josephson junctions.
 15. Thesuperconductor fabrication system of claim 14, further comprising:forming a disconnected pad in the first set of pads, wherein the firstset of pads comprises a first pad electrically coupled to the firstJosephson junction, and wherein the first pad is electricallydisconnected from the first Josephson junction to form the disconnectedpad, the disconnected pad rendering the first Josephson junctionunusable.
 16. The superconductor fabrication system of claim 14, whereinan electrical property of the first Josephson junction is modified suchthat the first Josephson junction no longer operates as a Josephsonjunction.
 17. The superconductor fabrication system of claim 13, whereinthe subset is selected based upon a measurement of a parameterassociated with each of the set of Josephson junctions.
 18. Thesuperconductor fabrication system of claim 17, wherein the resonancefrequency associated with a particular qubit is one member selected froma set of (i) a predicted resonance frequency calculated based upon themeasured parameter, and (ii) an actual measured resonance frequency ofthe particular qubit.
 19. The superconductor fabrication system of claim17, wherein the parameter includes a resistance associated with aJosephson junction in the set of Josephson junctions.
 20. Thesuperconductor fabrication system of claim 19, wherein the resistance isa normal-state resistance of the Josephson junction.
 21. Thesuperconductor fabrication system of claim 13, further comprising:forming a first set of protrusions formed on the first chip; and forminga set of bumps formed on the first layer of the second chip, the set ofbumps formed of a material having above a threshold ductility at a roomtemperature range, wherein set of bumps are configured to cold weld tothe first set of protrusions.
 22. The superconductor fabrication systemof claim 21, wherein the first set of protrusions is of at least onemember selected from a set comprising Gold and Platinum.
 23. Thesuperconductor fabrication system of claim 21, wherein the set of bumpsis of at least one member selected from a set comprising Indium, Tin,Lead, and Bismuth.
 24. The superconductor fabrication system of claim21, further comprising: forming a flip-chip assembly comprising thefirst chip detachably attached to the second chip using the cold weld,wherein a parameter of a Josephson junction inside the flip-chipassembly is tunable by disassembling the flip-chip assembly at the coldweld.
 25. A computer-implemented method to fabricate a quantumprocessor, the method comprising: forming a set of qubits, at least onequbit in the set of qubits formed in a flip-chip configurationcomprising a pair of chips, forming the pair of chips comprising:forming a first chip having a first substrate, a first set of pads, anda set of Josephson junctions disposed on the first substrate; andforming a second chip having a second substrate, a second set of padsdisposed on the second substrate opposite the first set of pads, whereinthe subset of the second set of pads corresponds to a subset of the setof Josephson junctions selected to avoid frequency collision betweenqubits in the set of qubits, a qubit in the set of qubits resulting froma Josephson junction in the subset of Josephson junctions.